1. Field of the Invention
This invention relates to compressors for heat pumps and is concerned more particularly with the lubrication of a compressor having moving parts exposed to a condensible gas constituting the working fluid.
2. Prior Art
A heat pump employing a condensible gas has a compressor to compress the gas, the high pressure gas then being passed through a condenser/heat-exchanger to condense the working fluid and to transfer heat to a heat output system, the working fluid, which is now a liquid under pressure, then being expanded and passed through an evaporator/heat-exchanger to absorb heat from an input system before returning in vapour form to the compressor. Many working fluids are known for such heat pumps; halocarbon refrigerants are widely used for this purpose because of their particularly suitable physical characteristics. Although, for small heat pumps, sealed compressor units, e.g. using a diaphragm-type compressor, are sometimes employed, for a larger heat pump it is preferred to use open compressors, for example of the piston and cylinder type or rotary compressors of the axial or centrifugal fan type. In such open compressors, there are moving parts which are exposed to the working fluid and these moving parts have to be lubricated.
In general heat pump working fluids such as halocarbon refrigerants are highly soluble in lubricating oil and the resulting change in the oil composition due to the dissolving of refrigerant can lead to a large reduction in the viscosity of the oil. The solubility of the working fluid in the oil increases with pressure and hence the greater the pressure of the working fluid to which the oil is exposed, the lower is the resultant viscosity. Increase of temperature normally reduces the viscosity of an oil but, when the working fluid is dissolved in the oil, the effect is more complex. Increase of temperature decreases the solubility of the working fluid. As a result, in general for any given working pressure, there is some temperature at which the viscosity is a maximum. Below this particular temperature, the increased solubility of the working fluid in the oil is the predominant effect giving decreased viscosity whilst above this particular temperature the viscosity decreases because the effect of temperature on the viscosity of the oil is the predominant effect. The temperature at which the viscosity is a maximum depends on the pressure. Due to the solution of the working fluid in the oil, there are very substantial changes in viscosity of lubricating oil exposed to such working fluid in the ranges of temperatures and pressures commonly employed in heat pump compressors. This has lead to problems in the lubrication of working parts in such compressors.